SUMMARY OF WORK The dedicated 48-processor cluster has been commissioned and software installed. Work on development of the 3d model including diffusion and transport processes on all cellular spatial and temporal scales is underway. This involves completely recoding the Monte Carlo algorithm to embed it hierarchically in a nested series of solvers handling different scales. Meanwhile, we have constructed two approximate models. One of these models stylized calcium release units on a two-dimensional grid, and appears able to account for a surprising range of behaviors of local calcium release events. This model has now been parallelized on the cluster, allowing statistics of large numbers of events to be accumulated. A second, non-stochastic model was developed which has been unexpectedly successful in accounting for the robustness and hormonal regulation of the pacemaking activity of sino-atrial node cells.